1. Field of the Invention
This invention relates to new, storage stable aqueous solutions or dispersions of polyurethanes containing alkoxysilane groups which dry to form cross-linked films, to a process for their preparation and to their use as coating compounds or for the preparation of coating compositions for any substrates.
2. Description of the Prior Art
Polyurethanes containing alkoxysilane groups have already been used in the form of their organic solutions for the preparation of moisture hardening coatings (see e.g. EP-A-158,893 or U.S. Pat. No. 4,222,925). One serious disadvantage of these systems is the use of organic solvents with the attendant disadvantages of environmental pollution, combustibility and odor. Another disadvantage lies in the sensitivity of these solutions to moisture. Even slight traces of water may cause instant cross-linking or gelling which renders the solution unusable.
In spite of the great sensitivity of the alkoxysilane groups to traces of water, there has been no lack of attempts to prepare polyurethanes containing alkoxysilane groups in the form of aqueous solutions or dispersions which can be used as environmentally safer coating compounds. EP-A-163,214, for example, describes aqueous dispersions of polyurethanes containing organosilane groups but it is pointed out in this prior publication that if the "silyl concentration" in the polymer is greater than 0.1% by weight there is a risk of premature gelling which is countered by keeping the pH of the dispersion above 7. Such a low concentration of cross-linkable centers is hardly suitable for ensuring the high cross-link density essential for the production of high quality surface coats and adjustment of the pH to values above 7 as recommended in the prior publication obviously renders the described dispersions unsuitable for the preparation of coatings which harden spontaneously at room temperature.
The disclosure of U.S. Pat. No. 3,983,291 and especially of U.S. Pat. No. 3,941,733 confirms this teaching of the prior art. In U.S. Pat. No. 3,941,733, for example, it is stated that the compounds are at their most stable at pH values of 6.5 to 9 and that curing of the surface structures prepared from the dispersions is preferably carried out under stoving conditions. It is obviously on account of the problems of stability which are recognized by the authors of U.S. Pat. No. 3,941,733 that the proportion of cross-linkable siloxane groups is kept very low in the specific polymers described in the examples. A low proportion of these groups not only results in a low cross-linking density, as already mentioned, but also means that stoving of the coatings is essential if cross-linking is to be achieved to any significant extent with the small amount of cross-linkable groups present.
It was therefore an object of the present invention to provide new solutions or dispersions of polyurethanes containing cross-linkable alkoxysilane groups which would be free from the above-mentioned disadvantages of the systems known in the art. The new solutions and dispersions should in particular be stable in storage and contain a high concentration of cross-linkable alkoxysilane groups so that they may be used for the production of high quality surface structures which will undergo cross-linking at room temperature.
It was surprisingly found that this problem could be solved by providing the solutions or dispersions according to the invention described below and the process for their preparation. Contrary to the teaching in the state of the art cited above, the solutions and dispersions according to the invention combine a high concentration of cross-linkable alkoxysilane groups with very high storage stability at pH values of 3 to 6 and enable high quality coatings to be obtained which will harden at room temperature.